1. Technical Field of the Invention
This invention is an open-tube water-depth sensor for use with oceanographic equipment in general. It has particular use with the depth measurement of arrays of acoustic sources as used in seismic exploration.
2. Discussion of the Prior Art
Oceanographers employ submerged sensors that are towed behind a ship during a marine survey. To make meaningful measurements at sea, the depth of the sensors in the water must be continuously monitored. In a more specific application, in seismic exploration, an array of seismic sound sources such as air guns or water guns is towed behind the ship. In the case of air guns, the guns are charged with air under a high pressure of about 2000 psi. Periodically, the guns are fired to create a thunderous shock pulse in the water. The shock pulse generates an acoustic wave field. The wave field propagates downwardly through the water and into earth layers beneath the sea. The acoustic waves are reflected from the sub-bottom earth layers, whence they return to the surface to be detected by hydrophones near the water surface. See for example, U.S. Pat. No. 4,908,801, issued Mar. 13, 1990 to the assignee of this invention. That patent is incorporated herein by reference as a teaching of one aspect the seismic art.
In that patent, it was explained that the depth of the air guns needs to be known accurately because the gun depth has a profound influence on the wave shape of the acoustic pulses. Therefore, each gun has a depth sensor attached thereto. The type of sensor generally used in the seismic industry, such as the TXC-D1 mentioned in the '801 patent, has a variable reluctance active element. I have found that variable reluctance depth transducers are unreliable for precision work. The response is non-linear and the element suffers from hysteresis. Furthermore, the shock wave that is created when the guns are fired, often damages or destroys the depth sensor. Although more sensitive and accurate devices are known, they are too delicate to be mounted on an air gun.
U.S. Pat. No. 4,726,315, assigned to the assignee of this invention, is also incorporated herein by reference as a teaching of the manner and techniques by which oceanographic devices such as arrays of air guns and other seismic equipment are towed behind an exploration ship at sea.
A type of depth sensor is disclosed in U.S. patent application Ser. No. 4,388,827, issued June 21, 1983 to C. E. Kinzer et al. Here, a tube, open at one end, is connected to a source of pressurized gas at the other end. The open end is located at the bottom of a liquid body. Periodically, a flow of pressurized gas is applied to the pipe so as to bubble out the open end of the pipe that is located at the bottom of the water. After the air flow is cut off, the back pressure in the pipe, due to the hydrostatic head, is an analog of the water depth. That method has the virtues of simplicity and linearity of response but it is useful only for making single measurements in a benign environment.